1. Field of the Invention
The present invention relates to a vehicle alternator which is provided with a case, which comprises a pair of brackets placed back-to-back, in which ribs are disposed, which define ventilation windows in the bracket body.
2. Description of the Related Art
FIG. 8 is a front elevation of a conventional vehicle alternator and FIG. 9 is a cross-section along line IX--IX in FIG. 8. The pulley and shaft in FIG. 9 have been omitted in FIG. 8.
This vehicle alternator comprises: a case, which comprises an aluminum front bracket 1 and a rear bracket 2; a shaft 6, which is disposed in the case and to one end of which a pulley 4 is secured; a roundel-shaped rotor 7, which is secured to the shaft 6; fans 5, which are secured to both sides of the rotor 7; a stator 8, which is secured to the inner wall of the case; slip rings 9, which are secured to the other end of the shaft 6 and supply electric current to the rotor 7; a pair of brushes 10, which move in contact with the slip rings 9; brush holders 11, which accommodate the brushes 10; a rectifier 12 in electrical contact with the stator 8; a heat sink 36, which is fitted into the brush holder 11; and a regulator 17, which is secured in contact with the heat sink 36 to regulate the magnitude of the alternating voltage generated in the stator 8.
The rotor 7 comprises: a rotor coil 13, which generates a magnetic flux when an electric current is passed through it; and a pole core 14, which is disposed so as to cover the rotor coil 13 and in which magnetic poles are produced by the magnetic flux generated by the rotor coil 13. The pole core 14 comprises a first pole core assembly 18 and a second pole core assembly 19 which are mutually interlocked. The first pole core assembly 18 and the second pole core assembly 19 are made of iron, and have claw-shaped magnetic poles 20, 21, respectively. Each of the adjacent claw-shaped magnetic poles 20, 21 is disposed such that they define prescribed interpole spaces, so as to prevent magnetic flux from leaking from between the claw-shaped magnetic poles, and to define a cooling ventilation passage to cool the rotor coil 13.
The stator 8 comprises: a stator core 15; and a stator coil 16, which is a conductor wound around the stator core 15 in which an alternating current is generated in response to changes in the magnetic flux in the rotor coil 13 resulting from the rotation of the rotor 7.
The rectifier 12 comprises: a diode 26, which converts the alternating current generated in the stator 8 to a direct current; and a heat sink 27, for dissipating heat generated in the diode 26.
A plurality of exhaust windows 29, which are defined by a plurality of ribs 28, are disposed around the circumference of the bracket body 80 in the front bracket 1. A plurality of intake windows 34 are disposed radially inwards from the exhaust windows 29. Bore holes 30 are disposed in four places around the circumference of the bracket body 80 at intervals of 90 degrees. Bolts 31 pass through these bore holes 30 and engage the rear bracket 2, and the rear bracket 2 and the front bracket 1 support the stator core 15 from both sides. Mounting holes 33 are formed in each of a pair of V-shaped mounting legs 32. Bolts (not shown) pass through these mounting holes 33 and engage an engine body (not shown), and secure the vehicle alternator to the engine body.
Like the front bracket 1, the rear bracket 2 also has ribs 28, exhaust windows 29, intake windows 34, and mounting holes 33.
In a vehicle alternator of the above construction, a current is supplied by a battery (not shown) through the brushes 10 and slip ring 9 to the rotor coil 13, and a magnetic flux is generated, whereby the claw-shaped magnetic poles 20 of the first pole core assembly 18 are polarized with a north-seeking (N) pole, and the claw-shaped magnetic poles 21 of the second pole core assembly 19 are polarized with a south-seeking (S) pole. At the same time, the pulley 4 is driven by the engine and the rotor 7 is rotated by the shaft 6, so that a rotating magnetic field is imparted to the stator coil 16 and electromotive force is generated in the stator coil 16. This alternating electromotive force is converted to a direct current by means of the rectifier 12, its magnitude is regulated by the regulator 17, and the battery is recharged.
The rotor coil 13 and stator coil 16 continuously generate heat while they generate power. At the same time, a fan 5 rotates to remove the heat generated by power generation, and in the front bracket 1 and rear bracket 2, cooling air passes in through the intake windows 34 and is expelled to the outside through the exhaust windows 29, as shown by arrows A in FIG. 9.
FIG. 10 shows a heat transfer circuit from the stator 8 to the cooling air passing through the exhaust windows 29. In the figure, T.sub.s is the temperature (K) of the stator 8, R.sub.1 is the heat resistance (K/W) between the stator core 15 and the insertion portion 35 of the front bracket 1, which is where the stator core 15 is inserted into the front bracket 1, R.sub.2 is the heat resistance (K/W) between the insertion portion 35 and the root portions 28a of the ribs 28, R.sub.3 is the heat resistance (K/W) of the ribs 28, and T.sub.a is the temperature (K) of the cooling air.
Some of the heat from the stator 8 is transferred through the above heat transfer circuit and is finally transferred to the cooling air. From this heat transfer circuit, the temperature increase in the stator 8 (T.sub.s -T.sub.a) is given by Q.times.(R.sub.1 +R.sub.2 +R.sub.3)=Q.times.R, where Q is the heat flux (W) of the stator 8.
In order to keep the temperature increase in the stator 8 as low as possible in a vehicle alternator of the above construction when the heat flux Q of the stator 8 is constant, it is necessary to reduce the value of the above heat resistance R.
The problem is that the bracket body 80 in the front bracket 1 and the bracket body 80 in the rear bracket 2 are made of aluminum, which does not have very high thermal conductivity, and the ribs 28, which constitute the window frames of the exhaust windows 29 are also made of aluminum, and therefore heat resistance R is great, the temperature of the stator 8 increases, and power output must be reduced.
Japanese Patent Laid Open No. (HEI) 7-170695 discloses a construction in which a heat pipe of high thermal conductivity is used to dissipate heat in a generator body, but the problem is that the heat dissipation portion, which dissipates heat from the heat pipe to the outside is disposed on the outside of the generator body and makes the body of the vehicle alternator too big.